Vulnerability Of Buildings to Bombs: Additional Thoughts After Oklahoma City

Vulnerability Of Buildings to Bombs: Additional Thoughts After Oklahoma City


In February 1993, a large terrorist bomb was detonated two stories belowground in a parking garage at the World Trade Center (WTC) in New York City. This detonation caused six deaths and more than 1,000 injuries. It also exposed many people inside and outside of the fire protection community to high-energy physical phenomena and their potential to cause property damage, injury, and death uncharacteristic of the vast majority of building fires.

In December 1993, Fire Engineering published a number of technical articles specifically addressed to the fire protection community to inform this community about the WTC bombing. Many of the issues this incident raised for building fire protection design and the requirements for fire service interaction with such incidents were addressed as well. I wrote one of those articles.1 This article explores the differences in blast damage and blast-induced fire damage that occurred in the WTC and Alfred P. Murrah Federal Building incidents and their implications on fire service performance requirements.


World Trade Center

A van parked on the B-2 level of the WTC underground garage and carrying more than 1,000 pounds of ANFO (ammonium nitrate plus fuel oil2) exploded a few minutes after noon on February 26, 1993. Several garage floors were destroyed; the Vista Hotel, above, and a commuter rail station, below, suffered significant damage. Smoke, appearing on the upper floors of one tower in an astonishingly short time, and concerns about structural viability forced the evacuation of the nearly 30,000 occupants of the twin-tower complex. When the smoke cleared, there were six dead, 1,000 injured, and about $1 billion worth of monetary loss–and a resolve that it would never happen again.

But it did!

Alfred P. Murrah Federal Building

On April 19, 1995, at 9:02 a.m., in Oklahoma City, Oklahoma, a truck carrying several thousand pounds of ANFO3 and parked on the street just outside the main entrance of the Murrah Building exploded. About 400 people, including a number of small children in a day care center, were in the building. Several days–and many courageous acts–later, the death toll reached 169, and more than 500 were injured. The building was demolished a few weeks later. Monetary losses are expected to approach $500 million. Damage to Americans` sense of security appears to be beyond measure.


There were two significant differences between the WTC and the Murrah Building bombings. Virtually all of the detailed differences in these two incidents directly result from these two basic dissimilarities.


World Trade Center. The explosive charge was inside the structure in a poorly vented space–an underground garage.

Murrah Building. The explosive charge was outside the structure in a significantly less restrictive–almost well-vented–location.

Impact on Structure

World Trade Center. The detonation caused no significant structural collapse. The vast majority of the failed structural components were directly damaged or destroyed by blast energy.

Murrah Building. The detonation caused a more than 50 percent collapse. The vast majority of the total structural damage occurred because critical supporting structures were damaged by the blast–an effect that could have been duplicated with a few very small, well-placed charges.


From the two fundamental differences in these two attacks, several important consequences resulted.

Injury/fatality ratios among victims. Injury/fatality patterns should be considered in light of the demographics of the site and the surrounding neighborhood. At the WTC, more than 100,000 persons were within a few blocks radius of the detonation location. Six persons perished–all in the WTC complex itself (the target) and all in the immediate vicinity of the detonation. Approximately 1,000 were injured, few seriously–virtually all of them were in the complex itself. Many were injured due to smoke inhalation.

However serious this event was, neighbors across the street in the World Financial Center heard the frighteningly loud explosion but suffered no injury or damage. A block or so away, the first indication of trouble was the sound of emergency vehicles headed to the complex. Even more telling, several people continued to work in the WTC for several hours after the blast.

In this attack, virtually all of the death and injury was caused by the blast and blast-induced fire effects. Far fewer than one percent of the at-risk “neighborhood” population4 was affected by the blast. The number of dead, in turn, was a very small fraction of the total of those involved.

Compare this with Oklahoma City. There, roughly the same number of persons were involved–169 dead and more than 500 injured for a total of about 700 (vs. about 1,000 in NYC). However, in Oklahoma City, this total was likely more than 10 percent of the at-risk population (as opposed to less than one percent in NYC). Moreover, the ratio of dead to injured was remarkably different–0.6 percent in New York City vs. 24 percent in Oklahoma City.

Types and locations of damage and injury. There are other interesting differences between these attacks. In Oklahoma City, many people at great distances–several blocks–from the detonation were injured by flying or falling window glass. In addition to glass damage, structural damage occurred to many buildings in the neighborhood. In many of these buildings, the damage was so severe that some were beyond repair and had to be razed. However, there are no reports that buildings` structural damage caused extensive injuries except at the Murrah Building itself.

At the WTC, all of the damage and injury occurred within 300 feet of the detonation. A great deal of glass was broken at the Vista Hotel, but no glass was reported broken outside of the WTC complex itself.

Both attack differences are very much evident in these damage and injury patterns. Building collapse greatly exacerbates the death rate among building occupants. Bombs detonated inside buildings generally are partially or totally contained by the building and do relatively little damage in the neighborhood.

Thus, both of these tragic incidents support the conclusion that the best defense strategy should first reduce the probability of collapse and, second, minimize the vulnerability of glazing to blast effects. Unless we are 100 percent certain that incident prevention is possible, we should routinely address these issues not only for buildings that are potential targets but also for the neighborhoods in which they are located. While law enforcement quietly and competently interdicts many potential bomb attacks, it is unrealistic to expect that there will never be another attack. In fact, the dominant lesson of Oklahoma City is that major attacks can happen again and without the apparent support of international terrorist organizations.

If we prevent building collapse and minimize glass injury in a bomb event, we must still deal with fire and smoke hazards stemming from the detonation.

Fire and smoke hazards. Explosion and fire are frequently incorrectly lumped together. Explosions in and around buildings do cause fires, but these fires and their consequences are often very different from building fires started by other means. Generally speaking, an explosion is far from the best way to initiate a major fire in an ordinary building. Recalling World War II, we heard of hundreds of bomb attacks in many cities. Virtually all of these attacks caused some type of fire. However, when the attack strategy was based on causing fires, a different bomb, an incendiary bomb, was used in the attack. This weapon caused many small but persistent fires and severely challenged fire services on the ground. From this experience, we see that high-explosive weapons were used primarily to break things; special incendiary weapons were used to burn things.

In explosions within and near buildings, the expanding hot gas clouds actually displace the oxygen-rich ambient air required to support combustion. Thus, several general attributes of explosion-induced building fires are observed.

First, the fires are at some distance from the point of detonation where the expanding gas cloud, mixing with the cooler ambient air, is less effective in producing local oxygen deprivation.

Second, the fires frequently involve ignition of some indigenous accelerant such as fuel in cars located on the street or in parking garages or gas from a fractured gas line. This type of ignition occurs more frequently than ignition of furniture, draperies, or wall materials. This feature increases the rate at which the fires spread and strongly influences suppression tactics.

Third, fires often come in groups of separated individual sources.

Fourth, external bombs rarely cause building fires, though they may cause ignition of cars near the detonation.

Fifth, smoke-transport mechanisms in internal explosion-induced fires are quite different and potentially more rapid than conventional smoke-transport mechanisms. The rapid movement of significant gas volumes through a building can carry combustion products as well as explosion products.

Sixth, it is likely that fighting explosion-induced fires will often be accomplished in the face of failed extant building fire protection systems such as sprinklers, alarms, and communication.

In summary, exterior bombs are likely to produce exterior fires and generally do not greatly exacerbate the bomb incident. Interior bombs, while not the most effective means of “burning” the targeted building, can cause multiple, persistent fires and greatly exacerbate smoke conditions throughout the building. Interior bombs produce far more serious fire and smoke hazards than exterior bombs. These general features were convincingly illustrated in the WTC and Murrah Building bomb incidents.


A bomb event may last one or two seconds, not even affording the one or two minutes to adopt defensive responses that often save lives in normal building fires. Though some bomb incidents, such as the Bishopsgate bombing in London, are accompanied by one or more warnings, one cannot rely on such an opportunity to prepare and defend.

With bomb attacks, your only defenses are those in place and working at the instant of detonation.

Therefore, when all of the issues presented above are integrated into a plan for a given building, they unfailingly point toward the adoption of passive and permanent defense measures–passive because moving parts, including people and electronics, require costly maintenance and care; permanent because cost amortization over the lifetime of the building is nearly always required to justify the cost of individual defense measures.

This approach, in effect, emphasizes measures (such as knowledgeably selected and firmly enforced vehicle standoff) to reduce the risk of building collapse and permanent shatterproof glazing (such as laminated glass) to reduce the most common form of injury.


The good news is that bomb attacks generally do not cause major building fires. The bad news is that internal bomb attacks, which are most likely to produce building fires, often produce multiple fires that must be fought under potentially exacerbated smoke conditions in buildings of questionable structural integrity.

The bottom line is that fire services responding to these incidents will find their role more demanding. They will fight more complex fires under very poor conditions in buildings that pose a significant, and often difficult-to-assess, threat. And, even before that job is over, they will often begin the long and agonizing tasks of extricating survivors and recovering the dead. Unfortunately, this is another lesson learned through the two national tragedies discussed here. n

© Lorron Corporation


1. Massa, Ronald J., “Vulnerability of Buildings to Blast Damage and Blast-Induced Fire Damage,” Fire Engineering, Dec. 1993.

2. The precise composition of the explosive could involve several specific nitrogen compounds and might even include elements to heighten the blast energy. The exact explosive is not essential to this discussion.

3. ANFO is a “homemade” explosive produced by mixing two common materials. Without strict quality control and similar detonators, it is difficult to compare the “power” of the explosive charges used in the two incidents discussed here. As a rule of thumb, ANFO generally is about 60 percent as powerful as TNT, and the effects of these bombs are generally related to their total ANFO weight.

4. The “at-risk population” in a bomb attack generally is all individuals located within a radius of the detonation where direct blast effects could produce injuries or where damage, such as falling or flying glass, could cause injury. As an illustrative rule of thumb, a 2,000-pound TNT detonation could easily involve a 1,000-foot radius “neighborhood”–or a total of about 75 acres. All individuals within this radius are potentially at risk.

RONALD J. MASSA has been president and technical director of security technology development activities for Lorron Corporation since 1984. For the past 10 years, he has been involved in blast postincident analysis, blast modeling field test programs, and bomb defense. He holds several electronic security systems patents, has published extensively on security topics, and has designed and planned a major blast test program for security glazing. Massa, who has several engineering degrees from Massachusetts Institute of Technology, previously served as president and CEO of Dynatrend, an engineering consulting firm he founded in 1971.

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